The extraction of tea material is well known in the art. For example green tea is typically extracted with hot or cold water to form a dilute extract containing soluble tea solids. This green tea extract can be concentrated to form a concentrated extract which is sold in frozen, refrigerated or dried form. This green tea extract can also be combined with other beverage ingredients such as fruit juice, nectar, etc., to provide beverages having at least some of the desired flavor and sensory characteristics of green tea.
Green tea extracts initially contain high levels of unoxidized flavanols, especially monomeric catechins such as epicatechin, epigallocatechin, epigallocatechingallate and epicatechingallate that impart a desired taste quality (astringency) to the tea beverage. Unfortunately, these catechin components (molecular weight of from about 200 to about 500) can be oxidized to higher molecular weight polyphenols, especially the theaflavins and thearubigins, in the presence of other components in the extract. These other components include metal ions (especially calcium, magnesium, manganese, aluminum, zinc and iron), certain partially oxidized organic intermediates (especially quinones) that are formed when the green tea is initially extracted, and dissolved oxygen. These metal ions in the extract act as a catalyst, and along with the quinones and dissolved oxygen, convert the catechins to oxidized polyphenols that impart a less desirable, lingering astringency to green tea beverages.
These oxidized polyphenols that are formed by the oxidation of the catechins can interact and react with other materials in the green tea extract, such as caffeine, protein, pectins and/or metal ions, to form even larger and heavier complexes that eventually precipitate out. As a result, the tea beverage turns from the desired pale green color to an unappealing brown color over time. More importantly, the tea beverage becomes cloudy, turbid and develops a visible precipitate within a few days.
This discoloration and precipitation of complexed materials in tea containing beverages is not visually appealing. Some consumers consider such beverages to be distasteful and "old". Moreover, where it is desired to provide "clear" beverages containing green tea, the fact that these green tea extracts can change color and become turbid is certainly undesirable.
Attempts have been made to remove these complexes from green tea extracts. These methods include changing processing conditions, especially temperature to cause precipitation, followed by centrifugation, filtration, and removal of the precipitate. Other methods include suspending and stabilizing the oxidized polyphenols. See for example, U.S. Pat. No. 4,051,261 (Jongeling) issued Sep. 27, 1977. Still other methods include using chemical and enzymatic agents to solubilize the insoluble components, or using solvents to extract the tea leaf, so that only the unoxidized catechins are extracted. Even after using these methods, the catechins will still be oxidized over time to the less desirable oxidized polyphenols. Also, when the green tea extract is incorporated into a beverage having non-tea materials such as juice, punch, and/or nectar, the beverage can turn an unappealingly brown color and can become "muddy" with time.
Another method that has been used to lower the level of oxidized polyphenols such as the theaflavins and thearubigins, and to increase the levels of desired catechins, as well as the desired amino acid theanine, is disclosed in U.S. Pat. No. 5,427,806 (Ekanayake et al), Jun. 26, 1995. In this prior Ekanayake et al process, green tea is extracted with an aqueous acid solution of erythorbic acid and/or ascorbic acid, plus citric acid. The acid extracted tea solution is then treated with gelatin and the resultant precipitate filtered out. This prior Ekanayake et al process removes some of the undesired oxidized materials as well as iron that can be present in the green tea extract. However, not all of the undesired components in the green tea extract are removed, including metal ions other than iron (e.g., calcium and magnesium), the theaflavins, and complexing components such as pectins and proteins. Accordingly, there is still a need for a process that can provide a green tea extract in which components contributing to oxidized polyphenols and other complexing components are minimized, reduced or removed so that the resultant extract has improved clarity and color over time.